Some construction vehicles include a traveling drive device for wheel driving and a working implement mounted on an anterior section of the vehicle, the working implement being driven by a hydraulic fluid supplied from a hydraulic pump. Among these types of construction vehicles are those in which a force generated when the working implement lifts an object to be carried will act upon the vehicle as a counteraction from the object. Wheel loaders and forklift trucks, for example, correspond to such construction vehicles.
Wheel loaders have an articulated type of working implement, inclusive of a bucket and a lift arm, on an anterior section of the vehicle. The wheel loaders apply a driving force to the wheels via the traveling drive device, with the bucket plunging into the object, then raise the bucket to excavate the soil. At this time, the wheels increase in ground contact pressure since the force that raises the bucket works upon the vehicle body as the counteraction from the object. As the ground contact pressure of the wheels increases, maximum frictional force between the wheels and the ground surface increases, so that when the bucket is being raised, occurrence of wheel slip tends to be reduced below a level at which the bucket is not being raised.
If the excavating counteraction is small and the driving force is large, however, the wheels may slip because of insufficient frictional force. In addition, the slipping of the wheels may involve scraping against the road surface to make it rough and thus to significantly reduce subsequent working efficiency of the vehicle. For example, in a wheel loader having a torque converter type of automatic transmission (hereinafter, this construction vehicle may be termed the conventional type of wheel loader), since the torque converter yields an increase in output torque with an engine speed, an operator predicts a magnitude of the driving force from parameters such as the engine speed, and performs excavation while balancing the extracting counteraction and the driving force appropriately for suppressed wheel slip. Skill is needed to balance the extracting counteraction and the driving force. Conversely to the above, if the driving force is too small, the bucket may fail to plunge into the object deeply. In this case, the amount of object that the bucket can scoop may reduce, thus the workload per unit time may also reduced.
While known traction control for automotive use would be applied to suppress the wheel slip generated when a construction vehicle is operated as above, occurrence of initial slip is likely to be unavoidable since the driving force is limited only after the occurrence of the slip has been detected.
In order to prevent wheel slip from occurring, therefore, Patent Document 1 discloses a wheel loader that includes traveling control means to detect a position of a bucket and an excavating counteraction (load), then calculate a rotational moment of a working implement, based upon the detected position and excavating counteraction, and limit a driving force according to the particular rotational moment.